1. Field of the Invention
This invention relates to a new aromatic polyimide and particularly relates to an aromatic polyimide which is superior in transparency, resistance to coloring and solubility in an organic polar solvent.
2. Description of Prior Art
It has been known to employ an aromatic polyimide for forming an electrically insulating protective film (i.e., interlayer insulation film) in electric or electronic material industries. For instance, Japanese Patent Provisional Publications No. 48(1973)-34686 and No. 49(1974)-40077 have proposed to prepare an insulating film of a solid element, passivation film and layer insulation film of a semiconductive integrated circuit by employing an aromatic polyimide in view of its excellent characteristics in heat resistance and insulating property.
However, aromatic polyimides of the prior art have drawbacks in that these are sparingly soluble in organic solvents. Therefore, a protective film of an aromatic polyimide is necessarily prepared by steps of forming a coated film of a solution of an aromatic polyimide precursor (i.e., aromatic polyamic acid) and subjecting the coated film to a heat treatment at a relatively high temperature for a long period of time for drying and imidizing. It is difficult in the prior art to prepare a protective film at a relatively low temperature with high reproducibility. Therefore, according to the prior art, electric or electronic parts to be protected the polyimide film are liable to deteriorate by the applied heat.
On the other hand, aromatic polyimides which are soluble in an organic polar solvent have been already disclosed in, for example, Japanese Patent Publication No. 57(1982)-41491. These aromatic polyimides, however, are generally colored yellow, brown or reddish brown and not satisfactory in transmissibility for visible rays. Accordingly, these polyimides are not suitable as a display element of liquid crystals, or a protective film of a light sensor and solar cell.